Accepted answer

A rather stupid solution, so please do add better answers:

$ mv ~/.sbt/plugins/build.sbt ~/Desktop
$ sbt publish-local

This will try to reload the plugins, but fantastically asking them to come in flavour Scala 2.9.2 instead of Scala 2.9.1. So it will end up will problems finding the plugins. Then go back:

$ mv ~/Desktop/plugins.sbt ~/.sbt/plugins/build.sbt
$ sbt publish-local

Re-downloads the plugins for Scala 2.9.1, and the GPG plugin is enabled. Voilà.

Edit: Never ever try to move the file into the base directory ~/.sbt/ – you will screw up so much.

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